Turbochargers for gasoline and diesel internal combustion engines are devices known in the art that are used for pressurizing or boosting the intake air stream, routed to a combustion chamber of the engine, by using the heat and volumetric flow of exhaust gas exiting the engine. Specifically, the exhaust gas exiting the engine is routed into a turbine housing of a turbocharger in a manner that causes an exhaust gas-driven turbine to spin within the turbine housing. The exhaust gas-driven turbine is mounted onto one end of a shaft that is common to a radial air compressor mounted onto an opposite end of that shaft. Thus, rotary action of the turbine also causes the air compressor to spin within a compressor housing of the turbocharger that is separate from the turbine housing. The spinning action of the air compressor causes intake air to enter the compressor housing and be pressurized or boosted a desired amount before it is mixed with fuel and combusted within the engine combustion chamber.
The turbocharger turbine section comprises a turbine wheel, a wheel heat shroud, and a turbine housing. The turbine inlet is positioned along an outer diameter portion of the turbine housing. The turbine is a centripetal radial, or mixed inflow device in that exhaust gas flows inward, past the wheel blades, and exits at the center of the housing's diameter. Expanded engine exhaust gas is directed through the exhaust gas manifold into the turbine housing. The exhaust gas pressure and the heat energy extracted from the gas causes the turbine wheel and attached shaft to rotate, which drives the compressor wheel.
The turbocharger compressor section comprises a compressor wheel, a backplate, and a compressor housing. The compressor inlet is positioned at the center of the compressor housing diameter. It is a centrifugal, or radial-outflow device in that the air flows outward, past the wheel blades, and exits at the outer diameter of the housing. The rotating compressor wheel draws ambient air through the engine's air filtration system. The blades of the wheel accelerate and expel the air into the compressor housing where it is compressed and directed through ducting to the engine intake manifold.
A central housing and rotating assembly supports the compressor and turbine wheel shaft in a bearing assembly or system. Seals separate the center housing from both the turbine and compressor sections.
Because the rotary action of the turbine is dependent upon the heat and volumetric flow of exhaust gas exiting the engine, turbochargers are often of reduced effectiveness when the engine to which they are coupled is run at a low speed. The reduced effectiveness is often labeled turbo-lag. In order to overcome turbo-lag when the heat and volumetric flow of exhaust gas is low, an electric motor is known for rotating the shaft and inducing the compressor to spin.
However, adding an electric motor to a turbocharger often entails adding length to the turbocharger. The additional length of the turbocharger corresponds to the length of the electric motor. It is, therefore, desired that a turbocharger construction be devised that combines the features of an electric assisted turbocharger while minimizing the additional length added by the electric motor.
Additionally, the electric motor placed within the turbocharger is vulnerable to oil contamination. Therefore, a need exists for preventing oil contamination of the electric motor. Finally, the presence of the electric motor and its rotor assembly that rotates at very high speeds (upwards of 200,000 revolutions per minute), creates balancing and stability problems. Therefore, a need exists for a turbocharger design comprising a rotor assembly that is balanced and stable during turbocharger operation.